The present invention relates generally to radiant tube burners, and more specifically, to radiant tube burners which vitiate the oxygen content of combustion air with re-circulated flue gas so as to minimize nitrous oxides (NO.sub.x) sufficiently and to allow the flue gas to be thoroughly mixed with the combustion air to reduce CO emissions. This invention further relates to methods for reducing the formation of nitrous oxides in a radiant tube burner assembly. Specifically, this invention relates to a method of reducing nitrous oxide and carbon monoxide emission levels by recirculating flue gases into the combustion air mixture and thoroughly mixing them prior to combustion.
The mechanism of combustion is not fully understood even today, and different burner assemblies have varying degrees of success at lowering nitrous oxides and after harmful emissions. There is a measure of unpredictability to this field which exacerbates the ever present and critical need for new methods or technologies which more efficiently and effectively minimize NO.sub.x emissions and other waste products for a radiant tube burner assembly. This need is critical because much of the energy required by industry comes from the combustion of hydrocarbons, and older methods and devices do not satisfy new government regulations. As political pressure continues to drive acceptable pollution thresholds lower, the ever present need for more efficient and cost-effective methods for reducing NO.sub.x emissions continues to increase.
Because a number of environmental problems, including acid rain, photochemical smog and tropospheric ozone (i.e., the greenhouse effect) have been attributed to the emission of nitrous oxides, governments have imposed ever stricter regulations on NO.sub.x emissions.
A major source of NO.sub.x emissions is the combustion of hydrocarbon fuels. Hydrocarbon fuels such as gas, oil and liquified coal are known to yield nitrous oxides during combustion. Emissions are especially high where the combustion occurs at higher temperatures and/or there is a large amount of fuel-bound nitrogen.
The components of nitrous oxides, nitrogen and oxygen, are inseparable from the process of fuel combustion. Combustion air comprises oxygen and nitrogen, and nitrogen is commonly found in the hydrocarbon fuel. As it is impractical, if not impossible, to prevent the formation of nitrous oxides by removing nitrogen from the reactants, one of two methods is commonly used to reduce or remove nitrous oxides during or after the reaction. First, nitrous oxides may be removed after the combustion process using selective catalytic or selective non-catalytic reduction. Second, the combustion process may be modified to minimize the formation of nitrous oxides, by employing staged combustion or low-NO.sub.x burners.
In a low-NO.sub.x burner, the formation of nitrous oxides during combustion can be mitigated by reducing the oxygen content in the combustion air (i.e., vitiation) and/or combusting the fuel and air at a lower temperature. By vitiating the oxygen content of the combustion air mixture, less excess oxygen is available in the combustion reaction for NO.sub.x formation. By minimizing the peak flame temperature, and the flame temperature generally, less excess energy is available for breaking stable nitrogen bonds and reforming oxygen-nitrogen bonds, a precursor to the formation of nitrous oxides. Both approaches are used to minimize the formation of NO.sub.x emissions.
The oxygen content of the combustion air can be vitiated, or diluted, with a gas lacking or low in oxygen. It is convenient and economical to re-circulate and mix the products of combustion, termed "flue gas" or "exhaust gas", with the combustion air to yield a combustible air mixture with a reduced oxygen content. The preferred oxygen content is determined on the basis of the fuel used or the desired temperature of the combustion reaction. As used herein, a mixture of combustion air, motive air and exhaust gas is also referred to as "vitiated combustion air" in connection with the present invention.
Many low NO.sub.x burners function by controlling the fuel and air mixture so as to create longer and more branched flames which provide a more consistent combustion temperature (i.e., an "even" burn) and lower minimum peak temperature. Where a vitiated combustible air mixture is employed, it is important that the combustion air and flue gas be mixed thoroughly so the oxygen in the mixture is evenly distributed. A thorough mixture allows the burners to better control the flame profile. As an added advantage, the production of unwanted waste products like carbon monoxide is also mitigated.
U.S. Pat. No. 4,828,483 discloses a regenerative burner which is intended to vitiate preheated combustion air with flue gas directly from a furnace. The lowered oxygen content of the combustion air is disclosed as helping to minimize the formation of nitrous oxides. The heat of the flue gas is used to preheat the combustion air, thus contributing to higher furnace efficiencies. This arrangement is directed, however, to a twin burner pair which draws flue gas directly from a furnace. Furthermore, it is not directed towards the use of a radiant tube burner assembly.
Although not relating to low-NO.sub.x burners, U.S. Pat. No. 4,359,209, describes the use of an air ejector, driven by a fan, to overcome pressure resistance and create a draw so that outside air flows, or are induced, into a plenum and to be mixed with hot flue gases. This mixture of flue gas and outside air is used to heat a recuperator which, in turn, preheats combustion air. In this instance, the combustion air is not vitiated. The problem of reducing nitrous oxides in a radiant tube burner assembly is not addressed.
The radiant tube burner of U.S. Pat. No. 4,800,866, employs a jet pump to provide combustion air and flue gas with a lowered oxygen content to the burners for reduction of NO.sub.x emission. The combustion air is forced into the recuperator by a fan. A stream of the total combustion air is accelerated through a nozzle to create a relatively negative pressure region around the nozzle to draw in flue gas returned to the exhaust leg by a connecting leg between the burner leg outlet and the recuperator inlet. This mixture is directed axially through a plenum towards the burner. The combustion air is preheated and then mixed with flue gases before entering the burner leg. This configuration, however, requires the use of all the forced combustion air to operate the jet pump and directs the forced combustion air and recirculated flue gas directly into the plenum. Such an arrangement does not provide sufficient time for the flue gas to mix throughly with the combustion air before reaching the burner leg, thereby presenting a potential problem of high CO levels. Furthermore this arrangement is only usable with forced combustion air systems.